Lettuce is unique among the major vegetables in its nearly exclusive use as a fresh, raw product. It is rarely found far from a salad or sandwich. It is occasionally used as a cooked vegetable and has even been used as a substitute for tobacco in cigarettes. Ryder, E. J., Leafy Salad Vegetables, AVI Publishing Company, pgs. 15-16 (1979).
Lettuce is in the Cichoreae tribe of the Asteraceae (Compositae). Id. at 22. Its Latin name is Lactuca sativa L. Lettuce is related to chicory, sunflower, aster, dandelion, artichoke, and chrysanthemum. Id. at 22-23. It is one of about 300 species in the genus Lactuca (milk-forming). Id. at 23. Cultivated lettuce is closely related to common wild lettuce, L. serriola Torner. Id.
There are six morphological types of lettuce: crisphead (iceberg), butterhead, cos (romaine), leaf, stem, and Latin. Id. at 47. The crisphead type is the most common in the United States, while butterhead and romaine types are popular in northern and southern Europe. Id. In the United States, California is the leading producer of lettuce. California produces crisphead, leaf and romaine lettuce. In 1995, California's cash receipts for crisphead lettuce amounted to about $987 million dollars. 1996 California Agricultural Resource Directory. Furthermore, also in 1995, California exported about $154 million dollars of crisphead lettuce to other countries such as Japan, Canada, the European Union, and Korea. Id.
Lettuce cultivars are susceptible to a number of diseases such as downy mildew, sclerotinia rot, botrytis, powdery mildew, anthracnose, bottom rot, corky root rot, lettuce mosaic virus, big vein, beet western yellows, and aster yellows, just to name a few. This diseases result in millions of dollars of lost lettuce crop throughout the world every year.
Downy mildew is a highly destructive diseases of lettuce grown at relatively low temperature and high humidity. Ryder, E. J., Leafy Salad Vegetables, AVI Publishing Company, p. 52 (1979). Downy mildew is caused by a fungus, Bremia lactacae Reg. Pale angular yellow areas bounded by veins appear on the upper leaf surfaces. Id. Sporulation occurs on the opposite surface. Id. The lesions eventually turn brown, and they may enlarge and coalesce. Id. The symptoms occur first on the lower leaves, but under ideal conditions may move into the upper leaves of the head. Id. When this happens, the head cannot be harvested. Id. Less severe damage requires the removal of more leaves than usual, especially at the destination. Id. In California, seven pathotypes of Bremia lactucae have been identified: Pathotype I, IIA, IIB, IIC, III, IV and V. (Michelmore, R. W., et al. 1996. Variation in California Populations of Downy Mildew in 1995. In Iceberg Lettuce Advisory Board Annual Report. California Iceberg Lettuce Research Advisory Board, Salinas, Calif. p.44-57). In addition to these major groupings of Bremia strains, many other novel isolates have been found throughout the State.
Corky root rot is believed to be caused by a species of Rhizomonas. One species of Rhizomonas that is commonly found to cause corky root rot is Rhizomonas suberifaciens.
Corky root rot symptoms include yellow bands on tap and lateral roots of lettuce seedlings. Guide to Leafy Vegetable Production In the Far West, Ron Smith, ed., California-Arizona Farm Press (1997). Yellow areas gradually expand and develop a green-brown color and develop cracks and rough areas on the surface area of the root. Id. The entire tap root may become brown, severely cracked and may cease to function. Id. Feeder root systems are reduced and damaged. Id. Roots become very brittle and break off easily. Id. Internal discoloration of the root may occur. Id. When the root is severely discolored, above-ground symptoms show up as wilting during warm temperatures, stunting and general poor, uneven growth. Id. In California, several strains of Rhizomonas suberifaciens have been identified. However, only strain CA1 has been shown to cause damage to roots of plants grown in commercial lettuce production areas.
There is a need in the art for improved lettuce varieties that exhibit resistance to both downy mildew and corky root rot, and exhibit vigorous growth, and increased weight and yield.